Equine Piroplasmosis: An In-Depth Review (AAEP 2010)

Equine piroplasmosis (EP), which is classified as a foreign animal disease, has made several appearances in the United States over the past few years, and it's causing concern at racetracks around the nation--the main locations at which EP has been diagnosed. But horse owners should be happy to know that their horses are likely not at risk for contracting the dangerous disease as long as their horses are being managed with industry-standard hygiene practices. Many of the EP cases diagnosed since 2008 were completely preventable, according to the panel members who presented during a special session on EP at the 2010 American Association of Equine Practitioners convention, held Dec. 4-8 in Baltimore, Md.

The session, moderated by Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, a professor in the Animal Population Health Institute and the section of population health at Colorado State University, gave practitioners a timely in-depth review on EP and its impact on the American equine industry.

Equine piroplasmosis is spread through the transfer of blood; naturally, it is spread by certain species of ticks, however according to Traub-Dargatz it can also be spread by the reuse of needles, syringes, and other blood-contaminated equipment that has not been sanitized between uses. Two distinct agents cause the disease: the hemoprotozoas called Babesia caballi and B. equi. After a U.S. outbreak of the disease in the 1960s, extreme measures were taken to eradicate EP and the country was declared EP-free in 1988, said Traub-Dargatz.

AAEP Convention Coverage

For more on what horse owners need to know about piroplasmosis in horses from Dr. Josie Traub-Dargatz, check out the video coverage from the 2010 AAEP convention.

Some horses infected with EP show few to no clinical signs, while others show various combinations of clinical signs that include fever, anemia, icterus (jaundice), and anorexia (the horse is off his feed). Some horses will also display colic with altered fecal consistency. The method of choice for diagnosing the disease is a laboratory blood test.

In August 2008 a Florida horse tested positive for EP, said Mike Short, DVM, equine program manager from the Florida Department of Agriculture. The investigation into the source of the disease found that several horses recently imported from Mexico (two of which tested positive for EP) likely carried the disease agent into the country.

Investigators diligently searched the horses for ticks and tested any that they found on horses or in closely surrounding areas, however they were not able to detect B. equi or B.caballi in the ticks. Finally, investigators determined that the likely means of transmission was the reuse of needles and/or syringes, and a practice called blood doping, Short said. All of the positive horses were involved in nonsanctioned horse racing, meaning that all the horses were likely managed using the less than optimal hygiene practices, he noted.

The Florida outbreak was well-controlled, Short added, with the final quarantines lifted in February 2009.

A very similar outbreak occurred in Missouri, beginning in June 2009. Again, the index case was a Quarter Horse that participated in nonsanctioned racing, said Angela Pelzel, DVM, a Western region epidemiologist with the USDA Animal and Plant Health Inspection Service. A total of eight horses were determined to be infected with EP, all of them sharing the same trainer as the index horse.

Extensive tick surveillance was conducted on the premises in Missouri, however only a few ticks were found and none were species capable of transmitting B. equi. Investigators believe the method of transmission of the disease agent was, again, the reusing needles and/or syringes among a group of horses and other less-than-optimal hygiene practices.

The only outbreak of EP that was determined to be spread by ticks was the outbreak on a large Texas ranch that produces horses for working cattle and other ranch activities. According to Pelzel, a total of 292 horses on the ranch tested positive for B. equi. After extensive testing of many horses that were sold and transported off the ranch prior to the EP diagnosis at the ranch, nearly 380 horses were determined to be B. equi-positive.

Authorities tested numerous ticks of several species from the property for B. equi, and it was determined that two of the types could spread B. equi between horses. It is believed that this is the only U.S. outbreak where the disease agent was spread by natural means in recent years.

Tracy Norman, VMD, Dipl. ACVIM, of Texas A&M University reviewed the activities surrounding the Texas EP outbreak and the ongoing efforts in Texas to plan for the need to evacuate or move positive horses for veterinary care. She also provided valuable information on how to thoroughly examine a horse for the presence of ticks.

The panel indicated that researchers are using this expansive outbreak as a learning experience, however. They said the ranch operators have been cooperative in quarantining their horses and allowing researchers to learn all they can about EP during the investigation.

Scientists have been working on several aspects of research related to EP, including refining the taxonomy (science of classification) of the disease agent, developing more advanced diagnostic tools, and finding treatment options to potentially eliminate the infection in positive horses. Researchers are currently evaluating imidocarb dipropionate for the treatment of B. caballi and B. equi infected horses. Donald Knowles, DVM, PhD, Dipl. ACV, research leader of the Animal Disease Research Unit of the Agricultural Research Service at the USDA, said that while imidocarb dipropionate has been proven an effective anti-babesial chemotherapeutic drug for the treatment of clinical signs of disease, some other potentially dangerous issues with imidocarb dipropionate need to be addressed before the treatment method becomes standard.

For example, Knowles said that it's been suggested that some treatment regimens with imidocarb dipropionate might cause imidocarb dipropionate-resistant strains of both B. equi and B. caballi, which would complicate control plans, as the goal in the U.S. is to eliminate the transmission risk, not just to resolve the clinical signs.

Equine pirolplasmosis remains a foreign animal disease in the United States, despite having several outbreaks on U.S. soil in the past four years. But most horses in the nation remain at a low risk of contracting the disease, the panel noted.

"Owners can obtain more information on how to protect their horses against infection from ... the USDA, from their State or Federal Animal Health officials or from private veterinarians," Traub-Dargatz added.

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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